The inductively coupled plasma (ICP) has been a powerful tool for analytical chemistry since it was introduced as an atomic emission source for optical spectrometry in the 1960's. In the ICP, radio frequency electrical energy is continuously coupled via a spiral load coil into an inert gas flow stream at atmospheric pressure. In a typical design, argon flows through a plasma torch made of three concentric quartz tubes (see, e.g., U.S. Pat. No. 3,958,883 (Turner)). The central gas flow, usually referred to the carrier gas flow because it is used to carry the sample to be ionized or excited, is usually 1 to 1.5 liters/minute, depending upon the characteristics of the sample. An intermediate gas flow of about 1 liter/minute, termed the auxiliary flow, is needed for confining the hot carrier gas and cooling. The outermost flow, termed the coolant gas, both sustains the plasma and protects the glass from melting from the high temperature. The coolant flow is generally 15 liters/minute with typical plasma torch designs.
The electromagnetic field induced by the radio frequency energy, which is typically at 27 MHZ or 40 MHZ, sustains a plasma in the gas. The plasma contains free electrons, ions, and excited atoms and molecules. A chemical sample, usually in a form of aerosol droplets, is introduced into the carrier gas steam through the central quartz tube into the plasma. The aerosol sample is vaporized and decomposed to atoms and small molecules, due to the high temperature of the plasma. Some of the atoms and molecules of the sample are further excited and ionized by the free electrons.
The ICP is a source suitable for coupling with an optical emission spectrometer (OES) since it produces a large number of excited and ionized atoms and molecules from a sample introduced into it (see, e.g., Wendt, R. H. and Fassel, V. A., Anal. Chem., 1965, 37, 920-922). In an optical spectrometer, the emitted light is usually sent to a wavelength dispersive grating and detected by a photodetector array. The output of the detector array is then electronically integrated for a certain time period. The ICP has also been coupled with a mass spectrometer (MS) (see, e.g., Houk R. S. et al., "Mass spectrometry of inductively coupled plasmas," Anal. Chem., 1980, 52, 2283-2289; and U.S. Pat. No. 4,760,253 (Hutton)). For mass spectroscopic detection, quadrupole mass filters are often employed. The chemical composition of the sample is determined by scanning the quadrupole filter within the mass range of interest. The description of the apparatuses and operation of ICP and spectrometers in Wendt, R. H. and Fassel, V. A., Houk R. S. et al., U.S. Pat. No. 3,958,883, U.S. Pat. No. 4,760,253, supra, and U.S. Pat. No. 4,818,916 (Morrisroe), infra, are incorporated by reference herein.
One of the major drawbacks of using the ICP for chemical analysis is the high cost of instrument manufacture and operation. To obtain useful radiation or to generate ions for routine chemical analysis, the ICP source is usually operated at high power, in the range of 800 to 1600 Watts, depending upon the sample introduced, which results in a very high temperature within to the plasma. Generation of such a high-energy radio frequency waveform normally requires a specially designed electronic circuit with high power output, such as the one described in U.S. Pat. No. 4,818,916 (Morrisroe). When the ICP source or the spectrometer (SP) in an ICP-SP system is not functioning normally, repair is often difficult and expensive. What is needed is an ICP-SP analytical apparatus with low energy consumption and that costs less to manufacture, operate and maintain.